A vast majority of specimens for histological studies are fixed, paraffin-embedded tissues. In order for sections taken from paraffin-embedded tissues to be stained or otherwise processed for examination and analysis, the paraffin must be removed from the sections. Early methods for deparaffinization employed flammable, volatile, and toxic organic solvents, such as xylene, to remove the parafin. However, safer, non-organic deparaffinizing agents are now commercially available. Further, U.S. Pat. No. 6,632,598 describes a deparaffinization solvent that is composed of non-polar hydrocarbons with boiling points between 140° C. and 250° C., polar organic solvents and surfactants. Non-flammable, non-volatile solvents are typically used in conjunction with heating to temperatures at or above the melting temperature of paraffin, approximately 50-57° C. For example, deparaffinization of a tissue section may be done by placing the slide with the tissue in an oven with resistance heating elements, a microwave oven, a pressure cooker, steamer, water bath or other thermal platform.
Specimens processed with formalin and/or other fixatives, and intended for certain immunohistochemistry (IHC) procedures, benefit from additional treatment to expose antigenic cites, referred to as antigen (epitope) retrieval, either during or after deparaffinization. Formalin is an effective fixative because it cross-links proteins making the tissue resistant to decomposition. But cross-linking can mask epitopes and prevent recognition of these sites by antibody reagents used in IHC procedures. Thus, the purpose of antigen retrieval processes is to unmask hidden epitopes, usually by way of a semi-destructive method.
Early methods of unmasking antigens make use of proteolytic enzymes to partially digest a tissue section. A major drawback with enzymatic digestion is that specimens tend to vary as to the amount of time needed for sufficient antigen retrieval, yet even a mild overexposure can lead to destruction of tissue morphology or loss of tissue from the slide and generally increases non-specific background interaction with antibody reagents.
Microwave heating has become a popular replacement for, or supplement to, enzymatic digestion to unmask antigens in fixed tissue sections. However, microwave heating is not ideal. Microwave ovens generally are not capable of uniform heat distribution throughout the heating compartment, increasing the risk that some slides may be under-heated while others are overheated. Microwave heating produces vigorous boiling leading to the evaporation of the liquid antigen retrieval reagent in contact with the slides, so typically, the method requires several rounds of microwave exposure interrupted by re-filling the containers holding the slides and treatment solution. Also, because microwave heating is difficult to control, tissue can be damaged and/or lost during processing.
Several automated instruments are commercially available that provide protocols for pre-treatment, including deparaffinization and antigen retrieval, and staining, but while these instruments allow different staining and IHC procedures to be performed simultaneously, they do not permit simultaneous performance of deparaffinization and/or antigen retrieval either together or with any other procedure. An automated antigen retrieval system, the “i1000™”, manufactured and sold by BioGenex (San Ramon, Calif.), is an exception, capable of simultaneously performing deparaffinization and antigen retrieval, but it uses microwave heating.
While such slide treatment devices often work well, they do have some disadvantages. For example, known slide treatment devices include robotic components and other moving parts, increasing purchase costs and maintenance efforts. Further, known slide treatment devices do not have an operating mode that prevents boiling of the deparaffinization solution or other liquid being used and thus, permit solution to be wasted in the boiling process and/or possible damage to the specimen. In addition, known slide treatment devices do not monitor the level of the solution in the device and may permit the level of the solution drops below the slides, which can adversely affect the slide treatment process.
Thus, there is a need for a slide treatment device is cost-effective, easy to maintain, and that has improved capabilities that overcome the disadvantages of known slide treatment devices.